Many research laboratories and manufacturers around the world are presently involved in the development of Laser Warning Receivers (LWR) for the purpose of protecting military platforms against laser guided weapons by detecting, identifying and locating laser sources associated with those weapons. A high angular resolution in the determination of the angle-of-arrival of the laser radiation from lasers associated with those weapons is essential in order to accurately locate those sources and optimize counter measures which can be effectively deployed against those weapons. The angle-of-arrival of laser beams can be determined by various techniques which can be classified into three different groups, i.e. imaging techniques, masking techniques and time-delay techniques. The present invention pertains to the group which uses masking techniques.
Basically, the masking technique consists of mounting a shadow mask containing one or several slots above an array of detectors. The angle-of-arrival of the radiation is determined from the position where the slot(s) is(are) imaged onto the detectors. For one dimension, a simple approach is to image a single slot on an array of narrow detectors aligned parallel to the slit, so that each detector covers a different sector of the scene. The main problem with this technique is that the number of detectors required, as well as the number of electronic channels, increases proportionally to the angular resolution wanted, thus leading to high costs in order to obtain a sufficiently high angular resolution. Using this type of technique, for example, 9 channels would be required to provide a resolution of 10 degrees within a field-of-view of 90 degrees.
U.S. Pat. No. 4,857,721 (David S. Dunavan et al) which issued on 15 August, 1989 describes, in a first embodiment, an apparatus for determining the direction of arrival of optical radiation wherein an elongated slit aperture is mounted perpendicular to elongated parallel detector strips arranged in an array extending parallel to the slit. An additional mask is located above the array and contains a number of opaque masked areas and a number of transparent areas directly above the detector strips which provide a Gray code pattern with different, complimentary, areas of alternate detector strips being opaque masked. The number of transparent areas above a detector strip increase from one end of the array to the other. A very narrow bar of radiation passing through the slit aperture will lie in a transverse direction across the encoded additional mask and, as a result, be detected by particular ones of the detector strips depending on if that narrow bar of radiation falls on an opaque or transparent area of the mask. The angle at which the radiation passes through the slit aperture will determine the position at which the bar of radiation falls on the encoded mask and it will fall on different transparent areas or opaque areas as the angle is altered. Therefore, the angle-of-arrival of the radiation can be determined from which detector strips detect the narrow bar of radiation. The processing electronics consists of a differential amplifier having its (-) input connected to one detector strip and its (+) input connected to an adjacent, complimentary, detector strip with the differential amplifier output being connected, by a coupling capacitor, to a comparator which provides a binary zero or a binary one code output. With 8 detector strips, this will provide 4 digital code output channels. It is possible, with this arrangement, to achieve a resolution of 6 degrees within a field-of-view of 90 degrees from those 4 channels when 5 transparent areas of the encoded mask lie above the last detector strip and one transparent area is located above the first detector strip and covers half the length of that first strip as illustrated in U.S. Pat. No. 4,857,721.
One disadvantage of the first embodiment described in U.S. Pat. No. 4,857,721 is the use of both an elongated slit aperture in a top mask and an additional mask having a Gray code pattern over the elongated detector strips. U.S. Pat. No. 4,857,721 also described other embodiments in which the additional mask is not required but in which each elongated detector strip is replaced by a number of detector elements (doped regions on a silicon wafer) with each element covering an area that corresponds with a transparent area of the additional mask in the first embodiment. These other embodiments have the disadvantage in that a large number of detector elements are required, each of which must be provided with electrical connections to the processing electronics.